1. Technical Field
The present invention relates generally to systems for tracking moving objects in conjunction with a laser beam and, in particular, to a self-referencing, imaging tracker that separately images target and target laser hit spot radiation to directly reference the laser hit spot relative to the target in a common coordinate system, thereby allowing the laser weapon to be locked onto, and maintained at, a desired target aim point until a target kill is achieved.
2. Discussion
Image trackers are often used in conjunction with lasers or other weaponry to disable inflight missiles. Conventional image trackers presently employ only non-self-referencing schemes for directing a laser beam to a desired target aimpoint. In practice, this means that the laser beam direction in space is inferred from the tracker line of sight as the tracker tracks the missile.
Trackers using imaging, non-self-referencing techniques typically utilize one or more imaging devices, such as electronic cameras, that first determine an approximate, or wide field of view (WFOV) position, and then an instantaneous, or narrow field of view (NFOV), position of a targeted object. After capturing the target image in the NFOV""s track gate, the trucker, under servo-loop control, follows the target. In most instances, the tracker is physically mounted on gimbals in a beam pointer. Therefore, the pointer line-of-sight also tracks the target if the pointer and tracker are properly boresighted.
Although conventional imaging, non-self-referencing trackers often provide adequate target location functions, a number of limitations exist with such systems. For example, in medium wave forward looking infrared (FLIR) based trackers, the laser weapon used for target engagement often interferes with the tracker imaging system, as instantaneous non-specular return from the laser hit spot on the object often blinds the camera, or, at least causes the camera automatic gain control to reduce camera gain to accommodate the bright laser hit spot, thereby losing all target image information. Typically, the laser-reflected power is some 40 to 60 dB greater than the target thermal signature. Additionally, with regard to long wave FLIR based systems, bright thermal energy from heated war heads may also blind such systems, causing the systems to lose track of the targeted object.
Solutions to the above problems include programming the system to select a laser aim point outside of the narrow field of view (NFOV) or the use of short wave infrared (SWIR) track bands with active illumination, causing the laser return to be invisible to the NFOV SWIR camera. If the laser aim point is selected outside of the view of NFOV however, the laser beam pointing must be determined by feed forward estimation. Such an end point selection is undesirable, as it eliminates missile nose-kill possibilities, and is subject to estimation noise as explained earlier. Alternatively, if a short range IR track band is used, the laser beam pointing must also be done via feed forward estimation. Such a scheme increases the susceptibility of the tracker to atmospheric disturbances.
Additionally, with non-self-referencing imaging trackers, the tracker line-of-sight must be accurately boresighted with the laser weapon line of sight. Due to the design of such systems, it has been found difficult to maintain an accurate bore sight under adverse environmental conditions.
Self-referencing trackers solve the above described limitations of the conventional imaging, non-self-referencing trackers by referencing the laser beam instantaneous position to the target image itself rather than to the tracker line-of-sight direction. Also, self-referencing trackers have lines-of-sight that need not be coaxial with the laser weapon, thereby subsequently minimizing the weight on the system gimbals and simplifying system transmit optics.
Presently, non-imaging self-referencing trackers, such as the systems disclosed in pending U.S. patent application Ser. No. 08/631,645, filed on Apr. 2, 1996, entitled xe2x80x9cLaser Crossbody Tracking System and Methodxe2x80x9d, now U.S. Pat. No. 5,780,838, and U.S. patent application Ser. No. 08/760,434, filed on Dec. 4, 1996, entitled xe2x80x9cLaser Crossbody and Feature Curvature Trackerxe2x80x9d, now U.S. Pat. No. 5,780,839, both incorporated herein by reference, are known in the art.
Non-imaging self-referencing trackers are presently deployed as vernier trackers; that is, the trackers correct residual image jitter created by imperfect image tracker performance. Thus, the non-imaging tracker bears the major tracking burden for difficult targets, such as small artillery rounds or maneuvering cruise missiles. Non-imaging self-referencing trackers use the laser beam itself to seek and hold onto a glint, such as a cylindrical missile roll axis. Therefore, the laser beam positioning on the target becomes independent of tracker jitter in the jitter direction and within the non-imaging tracker track bandwidth.
Although non-imaging self-referencing systems provide certain advantages over imaging, non-self-referencing systems, there is still room for improvement in the art. For example, there is a need for an imaging. self-referencing laser beam tracker that can be locked onto a desired target aim point, whether or not a glint is present at that point, and held on the aim point at will. In addition, there is a need for an imaging, self-referencing tracker that provides maximum noise immunity from atmospheric optical turbulence through measurement of the laser beam position relative to the position of the target through the same atmospheric path. There is also a need for an imaging, self-referencing tracker that reduces or eliminates the pointing error associated with the estimated aimpoint offset associated with conventional open loop trackers by measuring an actual laser hit spot location on the target relative to the target itself.
In copending application Ser. No. 08/919,413, filed on Aug. 27, 1997, entitled xe2x80x9cDichroic Active Trackerxe2x80x9d, now U.S. Pat. No. 6,021,975, a dichroic beam splitter is used to separate the near IR range radiation reflected from the target from the mid IR range radiation reflected from the high energy laser (HEL) to image the target and the target hit spot on separate detector arrays. Both the near IR range and mid IR range radiation reflected from the target are received over a common optical path. Unfortunately, there is about a 50-60 dB difference in the near IR radiation reflected from the target and the mid IR radiation associated with the HEL which can cause diffuse scattering of the reflected radiation which can result in saturation of the detector arrays possibly causing errors in the servo loop which locks the HEL on the hit spot on the target.
Accordingly, the tracker of the present invention provides target aim point selection and calculation of the track-point offset via a high power laser beam closed loop tracking system. There are two variants of the present invention; In variant 1, a pulsed near-infrared band illuminator laser, mounted near the tracker, illuminates the missile body, enhancing its image, as received by the tracker. In the tracker are two detector arrays; one sensitive to the near-IR radiation associated with the illuminator, but insensitive to the mid-IR radiation associated with the laser, and a second array sensitive to the mid-IR band but not the near-IR band in which the target image is determined. In the second variant, the mid-IR band is divided into two sub-bands; one containing all the laser lines, and one extending to the band limit determined by atmospheric absorption. Typically the bands are; (a) laser: 3-4 xcexcm; (b) target image; 4-4.5 xcexcm. These sub-bands are created by a blocking filter that is placed in the target image optical path. The image and laser hit spot from either variant are co-registered by optics choice and physical constraints placed on the detector arrays, then target and hit-spot centroid information refer to a common coordinate system from which the laser hit centroid vector distance to the image centroid is readily determined. The desired laser aimpoint may also be transcribed into that same coordinate system, therefore a servo-loop arrangement sensitive to the vector difference between the laser hit-spot and the laser aimpoint activates, driving the laser hit spot to the assigned laser aimpoint.
More particularly, variant 1 of the present invention provides a target tracker that includes a target illuminator that illuminates the target with radiation of a first wavelength. A laser weapon generates a laser beam comprised of radiation having a second wavelength. The laser beam engages the target and forms a laser beam hit spot thereon. An optics subsystem receives and detects both the illuminated target and the hit spot. A controller is programmed to steer the laser beam in response to the detected target and hit spot locations.
Variant 2 of the present invention provides a target tracker that includes an optics subsystem that separately images target radiation and laser hit spot radiation. A blocking filter incorporated into the optics subsystem ensures that only radiation at the target radiation wavelength passes to the first detector, while only radiation at the target hit spot wavelength passes to a second detector. The blocking filter obviates the need for the target illuminator utilized in variant 1. A controller then steers the laser beam generated by the laser weapon in response to the detected target and target hit spot locations.
In addition, the present invention provides a method of tracking a target. The method includes the steps of selecting an aim point on a target; illuminating the target with radiation having a first wavelength; engaging the target with a laser beam of a second wavelength to form a laser beam hit spot on the target; simultaneously imaging the illuminated target and the laser beam hit spot; and steering the laser beam to a target aim point based on a calculated difference between the target aim point and the laser beam hit spot.
In an alternate embodiment of the invention, a shared aperture is used for the outgoing high energy laser (HEL) and the incoming reflected radiation from the target by way of a telescope optics arrangement. A sapphire shared aperture arrangement is used to take advantage of the annular occlusion in the outgoing HEL beam. In particular, a hole in the shared aperture element, corresponding to the annular occlusion region in the HEL beam, transmits near IR range radiation from the target and reflects mid IR range radiation associated with the HEL beam. Since the millimeter wave infrared reflectance-coated sapphire transmits near IR range and reflects mid IR range radiation, the target detector is insensitive to mid IR radiation scattered from the target while the scattered light in the mid IR range is reflected to a mid IR range detector for imaging the laser hit spot on the target. In order to prevent the diffuse scatter from affecting the mid IR range imaging detector, the reflected radiation in the mid IR range is directed to a tilt mirror and, in turn, reflected to the laser spot detector array. Depending on the angular position of the mirror, the tilt mirror is images radiation from the laser hit spot on the target or from the background. By periodically sampling the background radiation, the background radiation can be subtracted from the mid-IR radiation in order to improve the signal to noise ratio of the signal. The target and laser hit spot detector arrays are co-registered to enable the laser to be locked onto an aimpoint on the target by closed loop control.